Blowing whistles and horns and waving flags and banners, which claim Mr Cameron's "Tory Fat Cats" are to blame for the cuts, the noisy demo chanted "Tories out!" as they began the march.

Organised by the Trades Union Congress (TUC), it has been billed as a march and rally for "The Alternative - jobs, growth, justice" in opposition to the coalition Government's cuts to public services and pensions.

Huge numbers of mainstream public sector unions and workers, including firefighters and teachers, are taking part along with a range of left-wing activists. Police are on alert for extremists attempting to hijack the peaceful union protest, with mounted police, vans and officers lining the march route.

Police have promised a "robust" response to anyone causing trouble, with a major operation under way to keep the peace.

A mass rally at the end of the march will be addressed by union bosses including Bob Crow, leader of the RMT, Mark Serwotka, from the PCS union and Len McCluskey, general secretary of Unite.